Temp Gauge Disco 2

I have following problem with my Disco 2 TD5 '99 - temp gauge isn't showing anything - just laying there plain dead. Is there any suggestions where I should check wiring for the temp sensor?

Gauge wasn't working even after ~50km / ~1h ride in quite hot day, but the engine is running smooth and at least based on my experience from other cars (not LRs) previously owned not overheating. Anyway - I just want to fick foking gauge before something bad happens and don't know where to start Suggestions r welcome!

earth wire on temp sender on front passenger side where top hose enters head see if guage goes to max .check engine through bung on top hose is full of coolant

Just to make sure - passenger side for LHD or RHD?

So - small update - engine is full with coolant - that I checked through bung on top hose. Regarding temp sender - in my version (LHD) it's probably located on drivers side where top hose enters head. After removing the silencer coating I managed to get to it and it's full of oil. Unluckily I don't have right now (I'm on country side without access to my workshop) any solvent except gasoline to clean it so I tried to do it with paper towels. While the plug was removed and after my cleaning I tried to earth it with wire, but gauge wasn't moving. Instead when the plug was removed, red temp light went on in dashboard meaning that somehow the temp sender is working because after reconnecting it everything was as it was. Now I let my Disco to run for a half an hour - probably after that I can tell what's happening.

Until now - any more suggestions what to do? Are there any fuses responsible for the gauge?

Update to the situation - I have cleaned all the contacts (temp sender, ECU) from oil, temp sender is giving out correct resistance (I have compared it to the table I have found in LR forums), but gauge is still dead. Could it be earth problem in instrument panel?